As one of traffic intersections, a loop intersection (ring road or runabout) is known. An intersection usually stands for a point where a plurality of roads meets together, and an intersection where a plurality of roads meets together along a circle or ring road is called loop intersection. The loop intersection is one-way, i.e., the loop road is one-way. A vehicle that enters the loop road from a given road connected to the loop road travels the loop road in a predetermined one direction and is allowed to go to any desired road.
A system is proposed for preventing the collision of vehicles in an intersection. In this system, a signal at the intersection is provided with a distance-measuring device, and a device to which the distance is to be measured is mounted on a vehicle that is going to enter the intersection. The distance-measuring device provided on the signal measures a distance between the signal and the other vehicle approaching the intersection, and an alarm is generated in the vehicle when the other vehicle approaches the intersection within a predetermined distance (for example, JP-A-2001-167395).
Another system is proposed for preventing a head-on collision that may happen as vehicles meet together by informing, in real time, all vehicles traveling on the roads and approaching the intersection of the position information and speed information of all vehicles traveling on the road and approaching the intersection (for example, JP-A-2001-143197).
In a loop intersection in many cases, trees are planted and signboards are erected at places surrounded by a loop road. With trees being planted and signboards being erected at places surrounded by the loop road, a driver of a vehicle entering the loop road cannot see the conditions of the loop road on the opposite side since his visual field is interrupted by the trees and the signboards.
Further, if two vehicles enter the loop road from separate roads, the vehicle subsequently entering the loop road may fail to notice the other vehicle that has already entered the loop road and is traveling thereon.